Frequency converter

[0001]FIG. 1 is a front, right side and top perspective view of a frequency converter comprising a new, original and ornamental design;

[0002]FIG. 2 is a front view thereof;

[0003]FIG. 3 is a back view thereof;

[0004]FIG. 4 is a top view thereof;

[0005]FIG. 5 is a bottom view thereof;

[0006]FIG. 6 is a left view thereof; and,

[0007]FIG. 7 is a right view thereof.

[0008]The broken lines depict portions of the frequency converter that form no part of the claimed design.